This invention relates generally to cathode ray tubes (CRTs) of the shadow mask type and is particularly directed to the measurement of electron beam landing errors of the type which cause degradation in color purity.
CRTs such as those used in television receivers and computer terminals are generally provided with a plurality of luminescent elements deposited upon the inner surface of the CRT's faceplate upon which a video image is displayed. Impingement of energetic electrons upon the luminescent elements, which are commonly termed "phosphor dots", results in light output from these luminescent elements. Near simultaneous illumination of large numbers of phosphor dots in a predetermined array results in the display upon the CRT's faceplate of a desired alphanumeric character or graphic image. Positioned immediately adjacent to the faceplate and within the CRT is a structure generally termed a "shadow mask" having a large number of apertures therein through which the energetic electrons transit as they are directed toward impact with the phosphor dots. In a color CRT, wherein three electron guns are positioned in close proximity to one another in the neck or rear portion of the CRT, each aperture of the shadow mask corresponds with a trio of phosphor dots on the faceplate which respectively emit red, green and blue light, the primary colors, when struck by energetic electrons. Each aperture in the shadow mask is aligned with the three electron guns and the three associated, grouped phosphor dots to permit only electrons from the red electron gun to be incident upon red phosphor dots. Similarly, electrons from the green electron gun illuminate green phosphor dots and the blue electron gun illuminates blue phosphor dots.
In order to avoid deleterious moire effects, a scanning electron beam from any one of the three primary color guns must simultaneously illuminate several dots of the appropriate color. If the green gun (usually the center gun in an in-line configuration) illuminates red or blue dots, a color contamination or loss of "purity" is said to result. Similar purity errors may occur with the other two primary colors.
Short of the condition in which, for example, the green gun illuminates portions of blue dots, a less serious "negative" beam landing error involving a loss of brightness occurs when an electron exiting a shadow mask aperture lands eccentric with respect to its associated phosphor dot. So called "negative guard band" color CRTs now in common use are designed such that electron beam cross section exceeds phosphor dot area. A portion of each electron beam is thus by design incident upon the non-luminescent guard region which surrounds each phosphor dot. The guard band is typically comprised of a graphite coating known as "black surround". A "positive" purity error occurs whenever the beam from an electron gun of a certain color illuminates the phosphor dot of another color. Frequently small beam landing errors in such negative guard band CRTs are imperceptible, even with microscope-aided visual detectors. For example, a slightly eccentric electron beam may fully illuminate its corresponding phosphor dot. Therefore, the display system designer seeking to accurately measure and minimize electron beam landing errors must consider not only the beam landing error of each beam with respect to its corresponding dot, but also the positions of neighboring dots of the two other colors.
As has been mentioned a non-luminescent "grille", sometimes referred to as "black surround", is printed on the inner surface of the CRT's faceplate or wherever it is desired to eliminate a light output. This guard region or grille not only reduces beam landing error (purity degradation) effects, but also serves to absorb ambient light incident thereon in order to improve video display contrast. However, there is a practical limit to reducing phosphor dot size while increasing grille area in attempting to improve contrast. Typically, beam current is increased in order to compensate for this limitation, but this approach also suffers from limitations as evidenced in loss of video image resolution.
Systematic electron beam landing or color purity errors may be minimized by proper design and fabrication of the lighthouse lens used in color CRT manufacture. This aspheric lens corrects for differences between light-optical ray trajectories involved in photo-chemical screen printing and electron-optical trajectories associated with the final product. An iterative process is used for lighthouse lens design, wherein with each iteration a reduction of beam landing errors is realized only if the lens designer has precise knowledge of previously measured purity errors.
U.S. Pat. No. 4,439,735 to Alvite et al, assigned to the assignee of the present application, discloses a method and apparatus for testing a line screen CRT for misregistration between its electron beam and the beam's phosphor stripe targets involving the sensing of light output at a plurality of test areas on the CRT screen as the electron beam is stepped across its phosphor stripe targets. The maximum and minimum light outputs of each test area as well as the beam locations at which the maximum and minimum light outputs were measured are used to compute the extent of electron beam misregistration with associated phosphor stripe targets for each test area on the CRT screen.
The present invention provides precise beam landing information to the CRT designer for accurate measurement and correction of beam landing errors in a system and method which allows for precisely controlled beam landing changes and accurate measurement of the resulting changes in light intensity. Beam landing data for the three primary colors is obtained in a matter of seconds in a self-calibrating, automatic system and method wherein the electron beam deflection current required to maximize brightness and minimize beam landing error on each axis is measured and corrected for optimum color purity.